1. Field of the Invention
The present invention relates to a method and an apparatus for washing the interior surfaces of tanks and containers. In particular, the invention relates to the cleaning of containers wherein a jet or a beam of cleaning fluid is ejected under high pressure and at a high velocity, which beam impinges the surfaces to be treated, and wherein the beams are controlled in particular with respect to their orientations with a view to cleaning or the like of predetermined surfaces interiorly of the container.
The cleaning by washing may be obtained through different effects, such as the dissolving effect of the washing fluid on impurities, loosening of dirt by the impact of the washing liquid or possibly of the cleaning particles slurried therein, or by heating of the impurities to render them more fluent or easier to dissolve through the influence of hot washing fluid.
In general, it is the object of the washing fluid to loosen adhering impurities and to transport them out of the tank, following which they may be processed, separated and/or disposed of in a controlled manner. Typical washing fluids include water with or without chemicals, oil products, solvents and/or mixtures thereof.
2. The Prior Art
U.S. Pat. No. 5,591,272 incorporated herein by reference discloses a method by which washing fluid is sampled from the usual contents of the tank, the washing medium optionally being roughly purified and heated to make it less viscous prior to its utilisation in the washing procedure.
Various washing heads are known on the market which are provided with nozzles and adapted to be installed in a fixed position and pivoted automatically to make the washing beam cover a specified solid angle during the washing process (corresponding to a surface section on a spherical surface) thereby ensuring that any point within this angle is covered with a guaranteed minimum intensity, and said units controlling the nozzles in accordance with various preprogrammed patterns so as to ensure that the distribution of the washing intensities in different directions are known. It is necessary for the nozzles to have degrees of freedom to pivot in two dimensions, i.e. in practice they should be allowed to pivot about two orthogonal axes. However, if the drive wheels for pivoting about the two axes are to be constructed in a mechanically uncomplicated manner, i.e. with simple mechanical gears, it is not readily feasible to provide uniform washing intensities in all directions in space seen from the nozzle's position. Thus, it is necessary to tolerate that usually some areas are more intensely washed than need be, if it is a priority to maintain a minimum intensity in certain other areas.
During cleaning of tanks the impurities to be cleaned out need not necessarily be uniformly distributed across the surfaces. In many instances a sedimentation has occurred which means that the tank floor may be covered with a thick layer of material which is difficult to remove. Another area where there may be a propensity to form solid deposits is the zone on the tank wall slightly above a liquid surface which has prevailed for an extended period of time in the tank where there may be a propensity to cake formation over time. In this case it is desired to direct a particularly high cleaning intensity towards the surfaces where the impurities have a particular tendency to stick or perhaps are particularly difficult to remove whereas other areas need not be subjected to an equally intense cleaning procedure.
In general, the tank geometry and the distribution of impurities relative to the positions in which the pivotable nozzles may be installed makes it difficult to match the beam pattern of the washing heads, and therefore general purpose washing heads with broad-sweeping beam patterns capable of covering all the directions to be reached, and washing for such extended period of time and with such intensity that in reality a substantial excess consumption of washing liquid occurs over a large portion of the tank are often resorted to. This excess consumption of washing liquid represents a poor exploitation of time, an increased energy cost, possibly an undesired wear on the tank interior, and it involves an increased cost of purifying the waste liquid which is discharged in larger quantities than desired.
U.S. Pat. 3,874,594 describes a washing unit including a nozzle arranged to be pivoted 360.degree. about a vertical axis and an angle about a horizontal axis to allow the washing beam pattern to cover a spherical surface, a rotatable head being driven by a shaft rotating centrally in a vertical support pipe, the rotatable head housing a worm gear arrangement causing a reduced speed rotation of the nozzle about the horizontal axis. The worm is free to slide axially a short distance equivalent to one half of the pitch of the worm in order that the nozzle may describe a helical pattern upon several revolution of the shaft, and upon reversing the direction of rotation, a non-coincident helical pattern during reversed rotation. The rotation of the shaft is driven by a turbine equipped with a gear box with changeable gears for reversal of the rotation. A lead screw mechanism in the gear box is connected to a cam mechanism associated with a lever adapted to control the pitch angle of the blades in the turbine.
The whole set-up looks exceedingly complicated comprising a great number of parts which must be matched very accurately and which indeed make it questionable whether this apparatus could be implemented in a practical version capable of actually operating as intended. The great number of parts, bearings and seals in contact with the washing medium represents a substantial complication, bearing in mind that the washing medium might include corrosive or aggressive ingredients and bearing in mind that any leakage in the area outside the tank are unacceptable in case oil or other inflammable liquids are used for washing medium. The rotatable nozzle head seems to be effectively suspended in the drive shaft representing a considerably complication in the manufacturing as well as in the maintenance work on the unit. The turbine and the presence of the shaft together with the various bearings inside the flow conduit are bound to cause a pressure drop in the washing liquid representing an energy cost for the pumping and a loss of washing effectiveness. Variations in the pressure in the washing liquid fed to the apparatus will influence the speed of rotation and the range of speed variations possible by controlling the blade pitch angle in the tubine will be narrow.
Patent application GB 2 096 455 discloses a tank washing apparatus with a washing head arranged to be pivoted 360.degree. about a vertical axis and an angle about a horizontal axis in order to allow the beam pattern to cover a spherical surface wherein the washing head is rotated about the vertical axis driven by a shaft arranged centrally inside the support pipe and wherein the rotatable washing unit includes means for causing the nozzle to pivot in a small increment about the horizontal axis by each revolution about the vertical axis.
The rotation is driven by means of a turbine rotated by the washing medium, the turbine driving a hydraulic pump connected by hydraulic connection lines to a hydraulic motor geared to drive the shaft. A lead screw mechanism is driven by the shaft and fitted with nuts which operate a hydraulic reversal valve in order to ensure the automatic reversal of the rotation.
This apparatus is quite complicated in including numerous small parts, bearings and seals, many of which are in contact with the washing medium and many of which will give rise to a pressure drop in the washing medium.
With a mechanism of a type wherein the orientation of the nozzle describes a helical movement pattern with parallel tracks disposed at completely identical intervals as seen on a spherical shell and wherein the speed of rotation about the vertical axis is constant, the beam ejection intensities are not identical in all directions. The nozzle allocates equal periods of time to angular paths of equal angular extent relative to the vertical axis. However, these angular paths correspond to solid angles of different sizes extending from small circles about the polar directions and to an expanded band around the equatorial plane. This heterogeneity may also be expressed in the angular velocity of the nozzle movement which approaches the angular velocity of the movement about the horizontal axis when close to the polar directions, whereas in the equatorial plane it is a vector sum of this velocity plus the angular velocity in the movement about the vertical axis. Therefore, a mechanism of this kind rotating at constant speed about the vertical axis will produce a beam pattern which is symmetrical about the vertical axis and wherein the intensity is higher in the axial directions than in the directions perpendicular to the axis.
In addition to being decisive for the intensity with which a given stretch or surface is swept, the angular velocity is of particular importance to the operational range obtainable with a washing nozzle. The liquid molecules which are ejected from the nozzles at a suitably high velocity will be slowed down when they strike on stagnant air. Thus, the ejection length obtained with a nozzle is most far reaching when the nozzle is set in a fixed direction thereby providing a liquid beam which continuously accelerates the air in an area around the beam path whereas the operational range of the beam drops if the nozzle is swept during washing because the liquid molecules in the front side of the beam will be slowed due to the air resistance.
It is considered realistic to obtain an effective cleaning effect at a distance of e.g. 25-30 meters from the nozzle outlet at an operational nozzle pressure of a magnitude of 12 bar and with the use of a suitably large nozzle, where the throughput amounts to 50-100 m.sup.3 per hour. However, this presupposes that the nozzle does not move or pivots only very slowly, the maximum allowable velocity being empirically expressed by a maximum travelling velocity of the beam's impingement area on a value comprised within the interval of 0.5-1.5 meter per second. If the velocity increases substantially beyond this limit, the beam loses its momentum by the slowing effect of the air, and it is scattered without obtaining said operational range. Although it is conceivable that the beam's impulse may be enhanced to increase the operational range by applying a higher operational pressure, increased volume throughput, etc., it will be understood that in case of ejection lengths of a magnitude of 25 meters, the air resistance will in any case severely restrict the sweep velocity of the beam.
Practice has established the need for cleaning tanks of particular configurations and with particular cleaning needs wherein the most desirable beam pattern is very different from the one which may be produced with the known washing heads. This applies to e.g. tanks where the roof is displaced vertically, e.g. the socalled floating-roof tanks where the roof floats on top of an enclosed amount of liquid during the normal use of the tank, and where the roof drops to a bottom position when the tank is emptied with a view to cleaning. In the bottom position, the roof is supported by supporting legs which serve to keep it in such a position that it is possible for the service personnel to enter the tank.
This constructive principle is employed e.g. in cylindrical tanks for the storage of oil where the tank diameter may be from 40 to 50 meters, occasionally as wide as 80 meters. In the empty tank the internal height above floor is typically from 1.8 to 2.3 meters. If the tank has a diameter of 50 meters it will be possible to sweep the entire tank floor where the most heavy impurities are located from a position at the tank centre provided that a washing nozzle having an operational range of 25 meters is employed.
Of course the nozzle will have to be arranged below the tank roof and the nozzle will have to be pivoted within a solid angle corresponding largely to a semisphere or a semispace below the nozzle whereby the entire floor area is covered. However, the intensity should not be the same in all directions from the nozzle within this semispace. On the exemplary assumption that the floor is to be cleaned by means of a nozzle located at a height of 1.5 meters above the tank floor, it may be calculated that within a nozzle angle of from 0 to 30.degree. from the vertical line, a subtending circle having a radius of about 0.9 meters and an area of 2.4 m.sup.2 would be covered, from 30 to 60.degree. a subtending circular belt towards a radius of 2.6 meters would be covered, the belt area being 18.6 m.sup.2, and from 60 to 90.degree. angle (it is assumed here that at 90.degree. the beam only just deflects and impinges the floor 25 meters from the nozzle), a subtending circular belt extending to a radius of 25 meters would be covered where the belt area is about 2,000 m.sup.2.
An apparatus producing a helical pattern and sweeping with constant speed about the vertical axis and with a constant speed about the horizontal axis will distribute even amounts of washing intensity to each of said three areas. If washing time is selected to produce the required dose of treatment in the outermost areas, it is estimated that the treatment dosage will be in the order of 100 times greater in the intermediate area and in the order of 1,000 times greater in the innermost area.